Basanite Glaciovolcanism at Llangorse Mountain, Northern British Columbia, Canada

Bull Volcanol (2007) 69: 329–340 DOI 10.1007/s00445-006-0078-1

RESEARCH ARTICLE

Margaret Harder . James K. Russell Basanite glaciovolcanism at Llangorse mountain, northern British Columbia, Canada

Received: 26 November 2004 / Accepted: 24 March 2006 / Published online: 23 May 2006 # Springer-Verlag 2006

Abstract The Llangorse volcanic field is located in Keywords Glaciovolcanism . Basanite . Lahar . northwest British Columbia, Canada, and comprises Palagonite . Thermal-model . British Columbia erosional remnants of Miocene to Holocene volcanic edifices, lava flows or dykes. The focus of this study is a single overthickened, 100-m-thick-valley-filling lava flow Introduction that is Middle-Pleistocene in age and located immediately south of Llangorse Mountain. The lava flow is basanitic in The Northern Cordilleran Volcanic Province (NCVP) composition and contains mantle-derived peridotite xeno- comprises Miocene to Holocene volcanic rocks distributed liths. The lava directly overlies a sequence of poorly sorted, across northwestern British Columbia, the Yukon Territory, crudely bedded volcaniclastic debris-flow sediments. The and eastern Alaska (Edwards and Russell 2000). The debris flow deposits contain a diverse suite of clast types, NCVP is dominated by small, mafic alkaline centres that including angular clasts of basanite lava, blocks of represent single eruptive events or cycles, although long- peridotite coated by basanite, and rounded boulders of lived, chemically evolved volcanic centres do occur (e.g., granodiorite. Many of the basanite clasts have been Mount Edziza, Hoodoo Mountain, Level Mountain). Many palagonitized. The presence and abundance of clasts of of the volcanic edifices and remnants that comprise the vesicular to scoriaceous, palagonitized basanite and NCVP display subglacial and ice-contact features, indicat- peridotite suggest that the debris flows are syngenetic to ing that these eruptions coincided with periods of the overlying lava flow and sampled the same volcanic continental glaciation (e.g., Mathews 1947; Edwards et al. vent during the early stages of eruption. They may 2002; Edwards and Russell 2002). represent lahars or outburst floods related to melting of a The Llangorse volcanic field (LVF) is approximately snow pack or ice cap during the eruption. The debris flows 144 km2 and is located approximately 55 km southeast of were water-saturated when deposited. The rapid subse- Atlin, British Columbia near Llangorse Mountain (Fig. 1). quent emplacement of a thick basanite flow over the Here, we present field observations and petrographic sediments heated pore fluids to at least 80–100°C causing descriptions for a basanite lava flow and associated in-situ palagonitization of glassy basanite clasts within the volcaniclastic deposits exposed near Llangorse Mountain. sediments. The over-thickened nature of the Llangorse This information is used to constrain the volcanological Mountain lavas suggests ponding of the lava against a origins of these deposits. Our study of this valley-filling down-stream barrier. The distribution of similar-aged lava and the underlying debris flow deposits suggests that glaciovolcanic features in the cordillera suggests the the lava and sediments result from a short-lived, synglacial possibility that the barrier was a lower-elevation, valley- volcanic eruption originating at a vent that has been wide ice-sheet. subsequently removed by glaciation.

The Llangorse volcanic field Editorial responsibility: H. Delgado

M. Harder . J. K. Russell (*) Aitken (1959) produced the first regional geology map for Volcanology & Petrology Laboratory, Earth & Ocean Sciences, the Atlin area and reported numerous remnants of young The University of British Columbia, volcanic deposits. The region lies entirely within the Vancouver, BC, V6T 1Z4, Canada e-mail: [email protected] Intermontane Belt and is underlain by Paleozoic sedimen- Tel.: +1-604-8222703 tary and volcaniclastic rocks from the Cache Creek Group Fax: +1-604-8226088 and by granitic to granodioritic plutons ranging in age from 330 Fig. 1 Geological map of Llangorse volcanic field (LVF; Map Sheet NTS 104N; after Aitken 1959 and Harder et al. 2003). The LVF includes occurrences of Miocene–Holocene volcanic rocks (Table 1) at: 1 Llangorse Mountain, 2 Table Hill, 3 Lone Point, 4 Hidden Ridge, 5 Hirschfeld Creek, 6 Chikoida Mountain, and 7 Fire Mountain. Inset shows the northern Cordilleran volcanic province (NCVP; shaded area) comprising Miocene–Holocene volcanic centres within northern British Columbia (BC), the Yukon territory (YK) and east- ernmost Alaska (AK) (Edwards and Russell 2000). Geographic reference points include White- horse (W), Atlin (A), Terrace (T) and Llangorse Mountain (LM)

Jurassic to Cretaceous (i.e., Harder et al. 2003). The 16 Llangorse volcanic field (LVF) includes Miocene to NCVP Phonolite 14 Holocene volcanic rocks exposed at Hirschfield Creek, Mt. Sanford, Llangorse Mountain, and Chikoida Mountain 12 Tephri- (Table 1; Fig. 1). This volcanic field largely overlies phonolite 10 Jurassic granodiorite of the Llangorse Mountain Batholith Trachy- O (wt%) and Cache Creek Group cherts (Aitken 1959) and many of 2 Foidite andesite Trachyte Rhyolite 8 Basanite the volcanic occurrences have been described previously O+K

(e.g., Nicholls et al. 1982; Higgins and Allen 1985; 2 6 Llangorse Mtn. Carignan et al. 1994; Francis and Ludden 1995). Lavas in Na Hidden Ridge Andesite the LVF include alkali olivine basalts, basanites, and 4 Lone Point olivine nephelinites (Fig. 2; Table 2; nomenclature after LVF 2 Basalt Table Hill Francis and Ludden 1990). The Llangorse Mountain Hirschfeld Creek locale, a Middle-Pleistocene volcanic remnant, comprises 0 olivine-phyric basanite lava (Fig. 1; Table 1). Other 35 40 45 50 55 60 65 70 75 relevant volcanic centres include Table Hill (olivine SiO2 (wt%) nephelinite), Lone Point (olivine-nephelinite), and Hidden Ridge (alkali olivine basalt; Figs. 1 and 2; Table 1). Fig. 2 TAS diagram (Le Bas et al. 1986) comparing chemical compositions of volcanic rocks from the Llangorse volcanic field (LVF) to those of the NCVP (open circles, light-gray shaded region). LVF sites include Llangorse Mountain, Table Hill, Lone Llangorse Mountain basanite Point, Hidden Ridge, and Hirschfeld Creek (see Fig. 1)

The focus of this study is the Llangorse Mountain locale which comprises a 100 m thick, 350 m wide bluff of basanite lava situated immediately south of Llangorse The lower margin of the lava flow is also marked by a Mountain (Fig. 1; Table 1). The lava flow features well- 30-cm-thick zone of vesicular, quenched basanite. Lava formed, 1–2 m diameter, vertically oriented columnar joints in this basal zone is aphanitic but not glassy (Fig. 4a,c) (Fig. 3a,c) and, at its base, the columns fan to define the and is characterized by pervasive, small planar fractures floor of the paleovalley that was filled by lava (Fig. 3b,d). that intersect to form crude polygonal shapes that average 331 Table 1 Locations and attributes of Miocene to Holocene volcanic rocks in the Llangorse volcanic field Locality UTMa northing- easting Rock typesb Volcanic landforms represented Volume (m3) Sourcesc

Llangorse Mountain 6582950–625950 Bas, mx > cx Valley-filling lava flow 2.1×107 1, 2, 4, 6, 7 Table Hill 6580122–619838 Ne, mx < cx Hypabyssal intrusion 3.0×106 1, 5, 6, 7 Lone Point 6577850–625600 Ne, mx < cx Subglacial lava flow 3.0×106 1, 5, 6, 7 Hidden Ridge 6585401–628415 Bas, mx cx Valley-filling lava flow 1.5×106 6, 7 Hirschfield Creek 6601100–616900 Ne, Bas, AOB Hypabyssal intrusion 2.0×107 1, 4, 5, 6, 7 Chikoida Mountain 6565200–612450 Bas, cx > mx Valley-filling lava flow >1.0×106 6, 7 Fire Mountain 6592000–625600 Ne, mx Dykes > 5.0×104 3, 5, 6, 7 aNorthing and easting in metres, relative to North American Datum 1927 bBas Basanite, Ne nephelinite, AOB alkali olivine basalt, mx mantle, cx crustal xenoliths c1 Aitken (1959); 2 Nicholls et al. (1982); 3 Higgins and Allen (1985); 4 Carignan et al. (1994); 5 Francis and Ludden (1995); 6 Harder et al. (2003); 7 Edwards et al. (2003)

~0.5–1.0 cm in diameter (Fig 4a). Commonly, small surrounding Llangorse batholith. The crustal xenoliths may (<1%) amounts of xenocrystic quartz are found in the represent surficial material incorporated into the base of the basanite along the same lower contact. lava during flow, which explains their relatively low The basanite lava is fine-grained (200–500 μm), equi- abundance and their size. granular, contains <5% microphenocrystic olivine and, Mantle-derived xenoliths (spinel-bearing lherzolites and generally, features a holocrystalline groundmass of plagio- harzburgites) are common at the base of the lava flow (~5 clase, clinopyroxene, and olivine, enclosing subordinate vol%). Most xenoliths are ≤5 cm in size but they amounts of Fe-Ti oxides and nepheline (Fig. 4b). Larger commonly range up to 10 cm in size. The largest xenoliths xenocrysts of olivine from disaggregated peridotite observed were approximately 25–30 cm in diameter. The xenoliths (see below) are characterized by thin (<50 μm) mantle xenoliths are extremely rare in the upper part of the reaction coronas or resorbed margins. Lava samples from flow, suggesting one of several possibilities: (1) the entire the highly fractured, quenched base of the flow (Fig. 4a) basanite bluff was molten at one time, allowing the dense show the same mineralogy as the main bluff, with one xenoliths to settle out of the upper part of the flow and important distinction. Primary magmatic biotite (≤2%) is concentrate at the base, or (2) peridotite xenoliths were found in all samples collected from the base of the flow only present in the early phases of the eruption, or (3) these (Fig. 4c). It is intergranular and appears as a late-stage and other xenoliths are accidental and entrained by the flow crystallization product; the abundance of biotite suggests during surface transport. that higher water contents attended crystallization in the We favour the first suggestion (1) of syn- to post- chilled base of the flow. emplacement sedimentation for several reasons. Firstly, it Crustal (granitic rocks) xenoliths in the basanite are rare is unlikely that the mantle xenoliths were incorporated into (<<1%), small (<1 cm), and appear to derive from the the base of the flow during surface transport because, other

Table 2 Major element chemical compositions of lavas (MH-02- ) from the LVF including replicate (r) analyses of individual samples. Localities include Llangorse Mountain (LM), Lone Point (LP), Table Hill (TH), and Hidden Ridge (HR) Sample 101 101r 101s 101s 112 160 173 213 Centre LM LM LM LM LM LP TH HR Rock typea Bas Bas Bas Bas Ne Ne Ne Bas

SiO2 45.13 44.99 44.91 45.00 44.11 41.47 40.57 45.57 TiO2 2.23 2.21 2.20 2.22 1.92 2.72 2.53 2.05 Al2O3 13.12 13.13 13.15 13.18 12.36 11.99 11.70 13.37 Fe2O3 2.23 2.59 2.80 2.73 2.63 5.37 6.70 2.76 FeO 10.09 9.68 9.59 9.59 9.76 8.35 7.57 9.58 MnO 0.18 0.18 0.18 0.18 0.18 0.19 0.21 0.19 MgO 11.69 11.61 11.65 11.68 13.55 12.02 10.67 11.04 CaO 9.96 9.96 9.94 9.99 10.24 9.18 9.45 9.37

Na2O 3.14 3.12 3.11 3.15 2.34 4.52 5.22 2.81 K2O 1.23 1.22 1.22 1.22 1.07 1.79 1.76 1.01 P2O5 0.50 0.50 0.50 0.51 0.43 1.00 1.30 0.38 Sum 99.51 99.19 99.25 99.45 98.59 98.61 97.68 98.13 aRock names as in Table 1 332

Fig. 3 Field photographs and interpretive sketches of outcrops at lines) and bedding attitudes in the sediments (dashed lines). c East- the Llangorse Mountain locale. a South-facing view of the valley- facing view of the basanite flow showing contact against underlying filling basanite flow. The bluff is approximately 100 m high and volcaniclastic sediments (top right corner of photo). d Sketch 350 m wide. b Sketch illustrating contact relationships between the illustrates contact, bedding, and columnar jointing orientations in c. basanite flow (B), volcaniclastic sediments (S), and granodiorite (G), Vertical rectangle represents schematic cross-section used in Fig. 7 orientation of columnar jointing within basanite flow (light dashed than the volcanic edifice itself, there is no immediate a highly weathered surface of granodiorite belonging to the source of peridotite and they are much more abundant than Llangorse Mountain batholith. The highly weathered the crustal xenoliths. Stokes settling velocity calculations nature of the granodiorite at this contact indicates that, at (based on a purely Newtonian fluid) suggest that even this elevation, the granodiorite was not glaciated. The smaller (5 cm diameter) mantle xenoliths will have settling eastern contact is only exposed near the upper surface of velocities of ~0.01–0.1 m/s (cf. Sparks et al. 1977). Such the lava flow and is nearly vertical in orientation. We settling velocities are likely to be at least an order of expect the contact between basanite and sedimentary magnitude slower than the rates of eruption. For example, deposits to lie at lower elevations, where it is obscured Kauahikaua et al. (2002) estimated eruption rates for the by scree sloughed from the lava bluff (Fig. 3a,b). The lack 1800–1801 eruption of olivine-phyric, xenolith-bearing, of exposure precludes direct measurement of thickness for alkaline basalts from Hualalai volcano to be in excess of the sedimentary deposits, however, based on our observa- 1 m/s. However, the rates of xenolith settling are tions we suggest a minimum thickness of 45–50 m. substantially faster than the time-scales required for transport, emplacement and, especially, cooling of this flow. The implication is that, once erupted, peridotite Volcaniclastic sediments xenoliths will immediately begin to sediment unless the lava maintains an excessively high stream-bed velocity. It The sedimentary deposits underlying the basanite flow is possible that the earliest phase of the eruption, as exposed at Llangorse Mountain comprise a sequence of represented by the base of the basanite lava flow, was consolidated, but still friable volcaniclastic sediments. The relatively enriched in mantle xenoliths and this could serve diamictites are very poorly sorted, crudely stratified, to explain the current distribution of xenoliths (e.g., matrix-supported, and contain angular to rounded frag- Kauahikaua et al. 2002). However, sedimentation rates ments (Fig. 5). The matrix of these sediments has a mean for mantle xenoliths in basanite liquid are sufficiently high grain size of 1–2.5 mm and there is a distinct lack of silt to that, even if evenly dispersed throughout the entire clay size fractions. Clasts or fragments vary in size from 1– eruption, most (>2 cm) xenoliths would settle to the base 5 cm to as large as 1–5 m. The stratification or layering is of a 100-m-thick flow within tens of hours to days. mainly defined by grain-size variations or by pronounced Along the western margin of the Llangorse bluff parting surfaces (Fig. 5a,b) and is decimetre scale; less (Fig. 3c,d; Fig. 5a), the basanite lava flow immediately common massive layers are metre scale. In some instances, overlies partially lithified, coarse-grained sediments. The layering is accentuated by the concentration of large contact between lava and underlying sediments appears (0.5–1 m) boulders at the top of the bed (Fig. 5b). The conformable. There is no evidence of post-depositional stratification is best exposed along the extreme southwest erosion or reworking of the sediments, nor is there portion of the outcrop (Fig. 3c). There the layering appears evidence for weathering or established vegetation on the primary and dips subparallel to the valley wall at contact. The same contact is obscured along the eastern approximately 25° to the northwest. side of the massif. There the basanite lava is in contact with 333

Fig. 5 Field photographs of volcaniclastic sediments underlying basanite lava flow at Llangorse Mountain. a Poorly sorted volcaniclastic sediments are charcterized by crude decimetre- to metre-scale layering (arrows point to people on the sediment/lava contact). b Alternate view of volcaniclastic deposits illustrating crudely bedded, poorly sorted, matrix supported, heterolithic nature of sediments. Light coloured material comprises rounded blocks of granodiorite

0.5–3 cm in diameter but some layers of diamictite are characterized by fragments of basanite that are up to 50 cm in diameter. Basanite clast types vary from angular to subangular fragments of dense lava (Fig. 6a) to subrounded Fig. 4 Textural and petrographic features of basanite lava. clasts of vesicular to scoriaceous basanite. Most are a Scanned image of polished sample from chilled base of lava vesicular, very fine grained, and commonly contain flow showing pervasive, irregular polygonal jointing. b Photomi- xenocrysts derived from granodiorite or peridotite xeno- crograph of holocrystalline basanite lava. c Photomicrograph of basanite from the chilled base of the lava flow overlying liths (Fig. 6b,d). In many instances, the basanite clasts have volcaniclastic sediments and containing small (<2 vol%) quantities the well-preserved shapes (fluidal) and textures (stretched of magmatic biotite (Bt) vesicles) of primary fragmented pyroclasts (Fig. 6). Fluidal shapes of pyroclasts and the presence of stretched vesicles (e.g., Fig. 6d) are fragile features formed by aerodynamic There are three main types of essential clasts within the shaping of low viscosity pyroclasts during eruption and are diamictites: basanite, granodiorite, and peridotite. These unlikely to survive high-energy stream sediment transport. clast types are always present even though their propor- The second most abundant clast type in these diamictites tions and size distributions vary throughout the section. is granodiorite which, on the basis of mineralogy and The matrix to the diamictites is dominated by angular to texture, probably derives from the surrounding Llangorse subangular clasts of basanite, palagonitized basanite and batholith. Granodiorite clasts comprise 5–20% of these secondary cement. Grains of quartz, feldspar, hornblende deposits, vary in size from 2 cm to 3 m in diameter (see and biotite are abundant throughout the matrix (Fig. 6b) light coloured blocks in Fig. 5b), and are generally well- and are produced by disaggregation of granodiorite clasts. rounded. The well-rounded nature of the granodiorite Olivine, clinopyroxene, and orthopyroxene grains are also clasts, relative to the angular to subangular volcanic clasts common constituents in the matrix and represent fragment- described above, requires a previous history involving ed peridotite. relatively well-worked or mature sediments. Glacial-fluvial The majority of clasts within the volcaniclastic sedi- sediments rich in granodiorite derived from the Llangorse ments are fragments of basanite. Most basanite clasts are 334

Fig. 6 Detailed textural and petrographic features of volcaniclastic from the Llangorse batholith. Vesicles and matrix pore space are deposits. a Scanned image of polished slab of moderately indurated partly filled by zeolite (Z). c Palagonitized pyroclast of scoriaceous sediments showing angular clasts of basanite lava (B) and basanite. d Palagonitized pyroclast of basanite showing gradation palagonitized basanite scoria (PB) within a finer-grained, poorly from a microcrystalline core (left) to a rim of palagonitized glass sorted, porous clastic matrix. b Photomicrograph of palagonitized (right). Vesicles in the palagonitized rim become increasingly clasts of vesicular basanite (PB) containing olivine (Ol) xenocrysts; stretched towards the clast boundary matrix contains quartz (Q), amphibole and biotite grains derived batholith underly much of the present-day landscape, and inated by smectite. Palagonitized basanite clasts can com- this is the likely source for the rounded granitoid clasts. prise up to 30% of the volcaniclastic sediment (Fig. 6a,b) The third essential component to the diamictites is clasts and vary in appearance from a bright orange, transparent of coarse-grained, mantle-derived peridotite (cf. Harder material to a dark red-brown, nearly opaque material and Russell 2006). They are a common accessory clast (<3 (Fig. 6). In some larger clasts, 1–2 cm thick rinds of vol%) and are relatively fragile. They tend to occur as palagonite encompass aphanitic, microcrystalline cores of friable, angular to subangular fragments and vary in size basanite (Fig. 6d). Palagonitized clasts vary in vesicularity from 0.5 cm to a maximum of 25 cm. Some peridotite from 5 to 55% (e.g., Fig. 6c) and, in all clasts, primary clasts feature thin coats of aphanitic basanite lava. porosity (vesicles) is partly to completely filled by Conversely, many of the basanite clasts contain small secondary zeolite (mainly chabazite) cements (Fig. 6b). fragments of mantle-derived peridotite xenoliths. Lava In the highly palagonitized portions of the deposit, zeolite flows containing mantle-derived peridotite nodules are coats most clasts and fills in pore space in the matrix, reasonably uncommon entities within the NCVP of causing lithification of the diamictite. northern British Columbia (Edwards and Russell 2002). On that basis alone, it seems probable that the xenoliths within the volcaniclastic sediments are closely associated Discussion with or related to the xenoliths within the base of the overlying basanite lava flow. Indeed, Harder and Russell Syngenetic debris flows (2006) used the peridotite xenoliths to investigate the thermal state of the underlying mantle lithosphere and A debris flow origin for the diamictite deposits underlying could find no systematic differences in mineralogy, mineral the basanite flow is supported by the poorly sorted, matrix- chemistry or texture between the two populations of supported, and crudely bedded character of the sediments xenoliths. Furthermore, the abundant well-preserved pyro- (e.g., Cas and Wright 1987; Geirsdottir 1990; McPhie et al. clasts of basanite within the diamictites also suggest a 1993). Such an origin is also strongly indicated by the size strong spatial and temporal connection between deposition distribution of clasts and the diversity in composition and of the volcaniclastic deposits and the overlying lava flow. shape of clasts. The lack of clay to silt size fractions in the Many basanite clasts are partially to completely matrix suggests a juvenile unweathered source such as palagonitized (Furnes 1984; Cas and Wright 1987; would be found on the slopes of an active volcanic edifice. Thorseth et al. 1991; Stroncik and Schmincke 2002) and Grain size distributions vary substantially between layers now are composed of a fine-grained mixture of clays dom- but the sediments commonly contain centimetre, deci- 335 metre, and metre-scale lithic blocks (e.g., up to 3–5 m). indicated by the presence of primary magmatic biotite These clasts derive from two distinct sources and have (Fig. 4c) and may have resulted from mixing of the substantially different shapes. The diamictite deposits underlying, wet debris flow sediments into the basal juxtapose well-rounded granodiorite clasts derived from portion of the lava (e.g., Zimanowski and Büttner 2002). pre-existing glacio-fluvial sediments against angular to This mechanical mixing between the unconsolidated fluidal-shaped juvenile pyroclasts of basanite from a sediments and overlying basanite could also account for primary volcanic source. the presence of small amounts of granitic material (e.g., The crudely stratified character of the debris flow quartz xenocrysts) concentrated at the base of the flow. deposits (Fig. 5a) could result from several disparate The debris flow sediments have many features indicative processes. The debris flow deposits may have been of their source region and transport distance. Firstly, the deposited en masse, in which case the layering might be sediments have an immature matrix, with few clay and silt- a manifestation of shearing at the base of the flow causing sized particles. Secondly, a high proportion of basanite sedimentation and some sorting (e.g., Cas and Wright clasts are fluidal-shaped or highly vesicular, and clasts of 1987; McPhie et al. 1993). Alternatively, the mass flows peridotite are common. These fragile juvenile volcanic may have been assembled progressively (Branney and features and friable peridotite clasts are rapidly destroyed Kokelaar 1992; McPhie et al. 1993) but with fluctuating or disaggregated by transport. The high proportion of rates of aggradation to account for the variations in the volcanic material in the sediments further indicates that the scales of stratification (e.g., decimetre to metre). Lastly, the sediments sourced near the volcanic vent. In particular, the diamictites may represent multiple episodes or pulses of abundance of glassy basanite clasts (now palagonitized) debris flow sedimentation. In this case, there would have indicates that the debris flows sampled material on or near been little to no hiatus between the mass flow events the volcanic edifice and supports a proximal, volcanic because there is no evidence of fluvial reworking or more source to the debris flows. dilute sedimentation at the tops of the individual layers Debris flows are commonly associated with volcanic (e.g., fine-scale lamination, graded bedding, cross-bed- eruptions (e.g., lahars, outburst floods) because of the ding). The attributes of these deposits are consistent with highly unstable and friable nature of juvenile volcanic high-energy flood events producing water-saturated material and steep topography often found in volcanic debris flows over short (hours to days) periods of time landscapes (Björnsson 1975; Cas and Wright 1987; (e.g., Björnsson 1975; Geirsdottir 1990; Knighton 1998; Geirsdottir 1990; Vallance 2000). Our interpretation of Vallance 2000). these debris flows, as lahars that are syngenetic with the A syngenetic relationship between the basanite lava and lava flow, suggests the production and release of the underlying debris flow deposits is indicated by the substantial volumes of water in association with the composition and character of the clasts within the volcanic eruption. A possible source for this water is the sediment. Most Neogene lavas in the NCVP are alkali- melting of a snow pack or glacial ice covering the vent. olivine basalt in composition and occurrences of basanite The high concentrations of juvenile materials in the lahars lava are relatively rare (Edwards and Russell 2002). The support the suggestion that the volcanic fragmentation basanite clasts in the sediments are petrographically (e.g., event(s) were actually coincident with melt water gener- mineralogy, texture) equivalent to the overlying basanite ation. Such events could include lava fountaining onto the lava. As discussed above, peridotite clasts from the snow and ice, or lava flow collapse on snow and ice sediments and peridotite xenoliths from the lava flow leading to generation of block and ash flows and debris show no discernible mineralogical or chemical differences, flow torrents (e.g., Thouret 1990; Smellie 2002). Contin- and appear to represent the same mantle source region ued eruption led to the production of a basanite lava flow, (Harder and Russell 2006). Therefore, we suggest that which was captured by the same drainage used by the there is a single source of peridotite-bearing basanite and debris flows. that this source supported the production of volcaniclastic debris flows as well as lava flows. The apparently conformable contact between the debris In-situ palagonitization flow sediments and the lava further suggests that deposition of the debris flows and emplacement of the lava flow An unusual aspect of the debris flow sediments is the were closely related in time. The highly fractured nature of presence of palagonitized clasts of basanite. Palagonitized the base of the overlying lava flow (Fig. 4a) is similar to the glass forms via the hydration and breakdown of volcanic small-scale, chaotic columnar or cube jointing found in glass (usually basaltic) in water-saturated environments, water or ice quenched lavas (e.g., Grossenbacher and and is a common feature of hyaloclastite deposits (Cas and McDuffie 1995; Lescinksy and Fink 2000). This type of Wright 1987; Stroncik and Schmincke 2002). Palagoniti- jointing is interpreted to result from steam generation that zation is most commonly found in subaqueous (submarine disrupts isotherms during cooling (Lescinksy and Fink or subglacial) deposits (e.g., Cas and Wright 1987; 2000). At the Llangorse locale, therefore, this jointing Stroncik and Schmincke 2002; Thorseth et al. 1991; pattern may indicate that the lava flow was quenched Furnes 1984) or in deposits subjected to hydrothermal against the underlying water-saturated sediments. Higher circulation of fluids (e.g., Jakobsson 1978; Jakobsson and water contents in the base of the lava flow are also Moore 1986). Early workers demonstrated that minimum 336 – 60 temperatures of 50 100°C are generally required for 25 10 5 2 1 B palagonitization to occur on time-scales of less than 0 1,000 years (Friedman and Smith 1960; Moore 1966). S Only high-temperature palagonitization processes pro- -60 duce smectite clays such as those found in the palagonitized basanite clasts at the Llangorse locale (Schiffman et al. -150 G

2002). Subaerial deposits that are subjected only to Depth (m) weathering do not undergo palagonitization, but produce 50 a completely different suite of clay and zeolite minerals -250 [Contours = years] (Schiffman et al. 2000, 2002). Higher fluid temperatures (a) 0 200 400 600 800 1000 (>100°C) are also conducive to the formation of authigenic o zeolite cements (Schiffman et al. 2000, 2002). Thus, the Temperature ( C) pervasive distribution of smectite clay and zeolite cemen- tation in the lahars from Llangorse Mountain supports a 600 (b) higher-temperature origin for the palagonitized basanite 0 m clasts. Further support for elevated fluid temperatures in 500 10 m Zeolite - these deposits is found in the colour of palagonitized glass. 400 Greenschist Dark-brown to red-brown palagonite is associated with 20 m Transition C) higher temperature fluids (80–120°C) than orange palago- o 300 nite (Stroncik and Schmincke 2001, 2002). Thus, the T ( 5-15 mm widespread distribution of dark-brown to red-brown 200 palagonite in these debris flow sediments (Fig. 7d,f) 100 suggests fluid temperatures in excess of 80°C. If the lahars 120 m derived from melting of snow or ice, they were probably 0 warm (20–40°C) at the time of deposition (Vallance 2000). 0 10 20 30 40 50 Regardless of the initial temperature of the debris flow Time (years) 0 sediments, the contemporaneous overlying basanite flow o C provided an efficient vehicle for heating the underlying 150 water-saturated deposits (Fig. 7). We suggest this process -50 o C led to hydrothermal, in-situ palagonitization of the glassy 100 -100 o basanitic clasts in the sediments. 75 C o 50 C We have tested this concept by modeling the heating of -150 25 o the subsurface by the basanite lava flow (Fig. 7). The C Depth (m) model also constrains the possible distribution (depth) and -200 timing of palagonite formation. We use a one-dimensional, transient, heat conduction model to simulate the cooling -250 (c) history of the basanite lava flow and the heating and 0 10 20 30 40 50 cooling history of the subsurface (e.g., Carslaw and Jaeger Time (Years) 1959; Philpotts 1990). The lava is given a thickness of 60 m (an average value to account for the wedge-shaped Fig. 7 Model heating and cooling history of volcaniclastic deposits underlying basanite lava flow. a Model results summarized as flow morphology) and is assumed to have a single uniform temperature–depth curves at specific times (years) showing relation- emplacement temperature (1,200°C). The upper surface of ship between cooling of 60-m-thick basanite lava (B) and heating of the lava cools to the atmosphere via natural convection and subsurface sediments (S) and granodiorite (G). b Temperature–time the base conducts heat to the ground which maintains a far- curves for heating and cooling of the subsurface based on depth field temperature of 10°C. The upper boundary condition (e.g., 0, 10, 20 .... 120 m) below the basanite-sediment contact. Conditions for palagonite formation are met where the heating– causes the upper surface of the lava to cool to solidus cooling curves intersect the minimum conditions (T°C - time) temperatures and potentially form a crust in a few hours; required to produce palagonite rind thicknesses of 5–15 mm (heavy this allows the effects of radiative heat loss to be ignored. curve). The high temperature limit to zeolite facies is also shown. Potential heat sources and sinks for the lava and the c The ’palagonite window’ (shaded field) resulting from heating of sediments by the overlying basanite flow is mapped in depth (m) vs. underlying stratigraphy were considered. However, our time space. Solid lines are isotherms (°C) showing the heating– calculations showed that the heats of crystallization within cooling history of the subsurface the lava are easily balanced by the heats attending phase changes in the subsurface (e.g., steam production and mineral transformations). On this basis and following the to the lava flow and underlying fluid saturated sediments work of others (e.g., Patino-Douce et al. 1990), we elected are assumed to be constant and independent of temperature. not to include heat sources or sinks. The pore water in the The boundary value problem was solved by finite- sediments is heated but the model does not allow for difference using fixed step sizes. convection. The last simplifying assumption is that the The transient temperature distribution in the lava flow bulk physical properties (e.g., thermal diffusivity) assigned and the subsurface are shown for a series of specific times 337 in Fig. 7a. The lava flow cools to <200°C within 25 years, and time limits, the temperatures of pore fluids would be while the subsurface (up to 50 m below the contact) is too low for efficient palagonite formation. heated to 100°C and is maintained at that temperature for 25 years (Fig. 7a,b). The contact is heated to approximately half the temperature of the lava flow (i.e., ~600°C) Glaciovolcanism in the LVF and maintained at temperatures >100°C for close to 50 years. At 20 m below the contact, the ground is The valley-filling basanite lava flow exposed at Llangorse gradually heated to temperatures in excess of 300°C but Mountain (100 m thick, 350 m wide; Fig. 3a) has an aspect cools back to ambient conditions after 50 years. These ratio (flow thickness/flow width) of approximately 0.3; low heating and cooling curves for the subsurface (Fig. 7b) can viscosity basanite lavas typically form sheet like bodies be used to explore where, in depth and time, they intersect with aspect ratios on the order of 10−4 (e.g., Reidel 1998). the conditions required for palagonitization of the debris Clearly, the Llangorse Mountain bluff is overthickened by flow sediments (Moore 1966; Friedman and Smith 1960; ~3 orders of magnitude. Moreover, the upper surface of the Schiffman et al. 2000, 2002; Stroncik and Schmincke lava is horizontal despite the 25° dip of the paleovalley 2002). floor, giving the lava a wedge-shaped cross-sectional No primary basanite glass remains in the debris flow profile (Fig. 3c,d). The wedge-shaped morphology and the deposits due to extensive palagonitization, which makes it overthickened aspect ratio of these lavas indicates that the difficult to estimate the maximum extent of palagonitiza- lava ponded against a lower-elevation, valley-wide barrier. tion. There are, however, basanite clasts that exhibit The present day valley below the Llangorse Mountain palagonitized rims surrounding microcrystalline cores bluff is a wide, glacially sculpted U-shaped valley that (Fig. 6f); the thickness of these palagonitized rims provides offers no current physical barrier. Large sheets of glacial ice a minimum estimate of the extent of palagonitization. In filling this valley at lower elevations would provide a likely Fig. 7b, we compare the model heating–cooling curves for barrier against which the lava ponded (Fig. 8; e.g., specific depths in the subsurface (solid lines; 0, 10, 20 m) Mathews 1951, 1952). This association of volcanic erup- to the temperature–time conditions (shaded curve) required tion with glaciation is typical of the NCVP, where there is to generate palagonite rim thicknesses of between 5 and abundant evidence of synglacial and subglacial volcanism 15 mm using the most recent rate laws (e.g., Techer et al. reflecting the coincidence of Pliocene to Pleistocene 2001; Stroncik and Schmincke 2002). Palagonitization is glaciers with volcanism (Edwards and Russell 2000). For predicted when the individual heating–cooling curves example, the Tuya-Teslin volcanic district contains numer- pierce this shaded curve. The upper temperature limits of ous subglacial volcanic edifices, including the type locality palagonite formation coincide roughly with the transition for tuyas (Mathews 1947; Edwards and Russell 2002). from zeolite to greenschist facies metamorphism (300°C; Evidence for glaciovolcanism is also preserved at Fig. 7b; Stroncik and Schmincke 2002). Thus, some paths several other volcanic localities in the LVF. The first is preclude the formation of palagonite because the heating the Lone Point locality (Fig. 1; Table 1) where a 200-m- path converts all palagonite and glass to higher temperature diameter-mound-shaped outcrop of nephelinite is exposed. mineral assemblages which will be preserved or altered to The Lone Point outcrop shows a complex pattern of other low-temperature assemblages (e.g., non-palagonite) columnar jointing, characterized by well-developed, 10– depending on the rates of cooling. 30 cm diameter columns with highly variable (i.e., fanning) These results lead to the concept of a ‘palagonite orientations. The second of these localities is Hidden Ridge window’ mapped in Fig. 7c. the palagonite window is (Table 1; Fig. 1) which features a ridge of basanite defined by the intersections of the model heating–cooling approximately 250×100 m in area. In overall morphology, curves (Fig 7a,b) with the critical T(°C)-t conditions for Hidden Ridge is very similar to the Llangorse Mountain formation of 5–15 mm rinds of palagonite (Fig. 7b). This bluff. The Hidden Ridge basanite is a valley-filling lava window (grey shaded field; Fig. 7c) shows the maximum flow that overlies a sequence of coarse, poorly sorted, depths and times for palagonitization of the subsurface crudely bedded debris flow deposits containing clasts of deposits compared to the isotherms (solid lines) predicted granodiorite, basanite lava, and peridotite. The geographic for the subsurface deposits. The isotherms illustrate the proximity and numerous similarities between Hidden maximum amount of subsurface heating and the critical Ridge and the Llangorse locale suggest that the two times where the subsurface begins cooling (Fig. 7c). For localities are roughly coeval in age and may have sourced example, the 100°C isotherm obtains a maximum depth of from the same volcanic vent. Hidden Ridge also displays 70 m, but after 35 years, the entire subsurface has cooled to variably oriented columnar joints, similar to Lone Point. temperatures below 100°C. Erratic size and orientations of columnar jointing in lavas at This simple model shows that the basanite lava flow at Lone Point and Hidden Ridge require both vertical and Llangorse Mountain had the capacity to heat the underlying horizontal cooling surfaces to explain the jointing patterns. debris flow sediments to temperatures of 100–125°C. The Strong variations in size and orientation of cooling joints model palagonite window suggests that this process would are also often indicative of interactions between lava and allow for formation of 5–15 mm rims of palagonite on ice masses (e.g., Mathews 1947; Jones 1966, 1969; glassy clasts in material up to 50 m below the contact and Grossenbacher and McDuffie 1995; Lescinksy and Fink would be complete within 30 years. Beyond these depth 2000; Edwards et al. 2002; Kelman et al. 2002), suggesting 338 basanite. Release of this melt water generated lahars which traveled down-valley, ultimately abutting and piling up Ice DF 2 against glacial ice. Portions of this drainage system may well have comprised tunnels and tubes within a large ice sheet. The basanite lava flow used the same drainage systems and overrode the lahars shortly after they were deposited DF 1 (Fig. 8b). The lava ponded against the valley glacier and continued flow resulted in inflation of the lava flow at its terminus (Fig. 8b; e.g., Cashman et al. 1998), ultimately a producing a 100-m-thick flow front (Fig. 8c). The ice-lava interface probably featured rubble that accumulated from the collapse of the pervasive, fine-scale, columnar to cube jointed flow fronts that characteristically develop in ice- Ice contact (quenched) lava masses (e.g., Mathews 1952; Basanite Lecinksy and Fink 2000; Edwards et al. 2002; Edwards and S Lava Russell 2002). The outer margins of the coherent lava flow would feature horizontal platy jointing, grading into polygonal, small-scale jointing, and ultimately into the large (metre-scale), vertical columnar joints observed today b in the centre of the lava flow (Fig. 8c). The present-day vertical lava bluff (dark solid line; Fig. 8c) lacks any trace of horizontal columnar jointing, suggesting that the leading edge of the lava flow has been removed. We suggest that as Basanite the glacier receded, the highly unstable and over-steepened front of the lava flow collapsed. Mass wasting of the front Ice Lava S of the overthickened, cliff-forming lava flow continues, forming a large apron of scree composed of blocks of columnar-jointed basanite, which surrounds the lava bluff and in most areas obscures the lower contact of the lava flow. c

Fig. 8 Interpretive sketch of the impoundment of the debris flow Conclusions sediments and basanite lava at Llangorse Mountain locale (vertical exaggeration >15×). a Early stages of eruption produce a sequence of debris flows (DF1, DF2, ...) ponding against glacial ice. The Llangorse volcanic field, in northwestern BC, b Subsequent lava flow is captured by the same drainage system, comprises Miocene-Holocene mafic, alkaline lavas. The overrides debris flow sediments, and is impounded against ice, focus of this study is a large, Middle Pleistocene, valley- causing inflation and overthickening (see arrows) at the flow front. filling basanite lava flow exposed south of Llangorse c Final post-eruption configuration includes a large overthickened mass of basanite lava; lava-ice interface is marked by accumulated Mountain. We interpret debris flow sediments underlying rubble (see text). Light dashed lines trace the orientations of the lava flow as syngenetic lahars or outburst floods that columnar jointing within the flow. Present-day exposure of the sourced from melting of a snow-pack or glacier around the basanite flow and debris flow sediments denoted by heavy solid line volcanic vent in the early stages of eruption. Volcanic (Fig. 3d is corresponding field photo) material was sampled near the vent and transported downslope in high-energy debris torrents. An effusive basanite lava flow soon followed the debris flows; the lava that lavas at these localities were erupted under or against ultimately entered a drainage system blocked at lower glacial ice. elevations by valley-filling glaciers, causing the flow to pond and overthicken to at least 100 m. Emplacement of the lava flow induced hydrothermal circulation of fluids in Volcanological model for Llangorse mountain the underlying, water-saturated debris flow and resulted in in-situ palagonitization of the glassy pyroclasts of basanite. The relative sequence of events producing the stratigraphic This rapid, high temperature (80–100°C) palagonitization relationships observed at Llangorse Mountain are schemat- also produced authigenic zeolite cements which partly ically illustrated in Fig. 8. The paleo-landscape consists of indurated the debris flow sediments. Our simple thermal pre-existing drainage systems cut into granodiorite bedrock model shows that the basanite flow heated the underlying and leading to glacial ice sheets present at lower-elevation ground (debris flow sediments and granodiorite country regions. The first stage of the eruption sequence (Fig. 8a) rock) to temperatures in excess of 100°C and maintained features contemporaneous production of large volumes of these temperatures for up to 50 years. Lastly, on the basis of water from melting of snow or ice by the eruption of thermal models and experimentally determined rates of 339 palagonitization, we develop a ‘palagonite window’ which Harder M, Russell JK (2006) Thermal state of the mantle lithosphere maps the depth (50 m)/time (30 years) limits for in-situ beneath the Northern Cordilleran Volcanic Province, British Columbia. Lithos 87:1–22 palagonitization of basanite clasts within the Llangorse Harder M, Russell JK, Anderson RG, Edwards BR (2003) Llangorse Mountain debris flow sediments. volcanic field, British Columbia. Geol Surv Can, Curr Res 2003-A6 Higgins MD, Allen JM (1985) A new locality for primary xenolith- Acknowledgements Field costs for this study were covered by the bearing nephelinites in northwestern British Columbia. Can J Geological Survey of Canada, courtesy of the Atlin Integrated Earth Sci 22:1556–1559 Geoscience Project (R.G. Anderson). All other costs were met Jakobsson SP (1978) Environmental factors controlling the through NSERC via a Discovery Grant to J.K. Russell and a PGS-A palagonitization of the Surtsey tephra, Iceland. Geol Soc scholarship to Margaret Harder. Our research benefited from Denmark Bull 27:91–105 discussions with Kirstie Simpson, Ian Skilling and Ben Edwards, Jakobsson SP, Moore JG (1986) Hydrothermal minerals and and the manuscript was improved substantially via critical and alteration rates at Surtsey volcano, Iceland. Geol Soc Amer thoughful reviews by Dave Lescinsky and an anonymous reader. 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